The field of the invention is nuclear magnetic resonance imaging ("MRI") methods and systems. More particularly, the invention relates to the suppression of signal from fat by optimizing the chemical shift selective ("CHESS") MRI technique.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The majority of commercial MRI systems excite and image hydrogen nuclei. Hydrogen is present in the human body in many different molecules, and due to the different molecular level interactions, the Larmor frequency of the hydrogen is shifted in frequency. This "chemical shift" of the Larmor frequency results in a spectrum of frequencies in the acquired NMR signals. For example, in an MRI system with a polarizing field of 1.5 Tesla the major NMR signal component produced by hydrogen in fat molecules is shifted about 220 Hz from the signal produced by hydrogen in water molecules. This chemical shift is often expressed independently of field strength as 3.5 parts per million.
In some applications it is desirable to produce images only of the water molecules. In the article "H NMR Chemical Shift Selective (CHESS) Imaging", Haase, et al., Phys. Med. Biol., 1985, vol. 30, No. 4, pp. 341-344, a technique is described in which the undesired signal component (e.g. fat) is first excited by a 90.degree. RF excitation pulse which selectively saturates the spin magnetization prior to each imaging pulse sequence. The longitudinal magnetization of the undesired spins is thus maintained at a low level throughout the image acquisition and the NMR signals which they produce are suppressed. The CHESS sequence is comprised of an RF excitation pulse which is frequency selective to the undesired spin Larmor frequency, followed by a spoiler gradient pulse which dephases the resulting transverse magnetization prior to commencing the imaging pulse sequence.
In most applications of the CHESS method the fat signal to be suppressed is assumed to be a single signal component offset in frequency from the desired water signal. The frequency selective saturation pulse ("ChemSat pulse") produces a 90.degree. flip angle for this signal component. In the publication "Optimization of Chemical Shift Selective Suppression of Fat", Proc., ISMRM, 6th Annual Meeting, pg. 1981 (1998) K. Kuroda, et al. describe a technique for calculating an optimal flip angle that takes into consideration the many different signal components in fat signals. While this work establishes that the flip angle can be optimized for the multiple fat signal components that are to be suppressed, it does not take into consideration the particular scan parameters used to acquire the image data.